Nerve stimulator output control needle with depth determination capability and method of use

ABSTRACT

An improved nerve stimulator needle which allows for improved ease of use and efficacy in the performance of targeted drug delivery to nerve. A variable control mechanism is contained in a housing to which a needle is attached and several electrical conductors are attached. The housing contains an embedded fluid path through which a syringe is attached to the needle. The variable control mechanism and the housing allows for positioning the needle and applying current to the needle with the same hand. The needle may also contain a linear resistive coating that enables the determination of the depth of the needle by determining the resistance of the length of the needle above the skin surface of a patient. Another embodiment of the needle may include an optical variable control mechanism.

[0001] This application claims priority to U.S. provisional application60/253,064 filed on Nov. 24, 2000, which is herein incorporated byreference.

TECHNICAL FIELD

[0002] This invention relates to a nerve-stimulating needle withfingertip control of stimulator current output and visual as well aselectrical depth determination ability.

BACKGROUND OF THE INVENTION

[0003] Anesthesiologists commonly use nerve stimulators and insulatedneedles for the purpose of locating peripheral nerves, or nerveplexuses, for the performance of regional anesthesia procedures. Thisprocedure is discussed in Vloka J D et al., “A National Survey OnPractice Patterns In The Use Of Peripheral Nerve Stimulators In RegionalAnesthesia,” The Internet Journal of Anesthesiology, Vol. 3, No. 4,1999. In addition to targeting local anesthetic delivery for regionalanesthesia, the use of nerve stimulators reduces the potential for nerveinjury since direct contact with the nerve is not required forperformance of the anesthetic. This is in contrast to the alternativemethod of seeking paresthesias to confirm needle position. Paresthesiasare provoked by directly disturbing the nerve. If such needle to nervecontact can be avoided, direct needle trauma should be reduced.

[0004] The available nerve stimulators have differing outputcharacteristics. The shape of the pulse is typically square or nearlyso. The pulse widths vary from 40 microseconds (μs) to 2 milliseconds(ms). Frequency selections for these stimulators range from 1.0 Hertz(Hz) to 100 Hz, in step gradations rather than continuously. Stimulatorsthat are manufactured specifically for regional anesthesia applicationstypically do not have frequency options greater than 5 Hz. Thestimulators that serve as both nerve stimulators and neuromuscularblockade monitors offer higher frequency choices, typically 50 and 100Hz. These stimulators are discussed in Barthram C N, “Nerve StimulatorsFor Nerve Location—Are They All The Same?,” Anaesthesia, Vol. 52, 1997,pp. 761-764.

[0005] The needles used for stimulator assisted regional anesthesiatypically have a molded plastic hub that contains both a connection toplastic tubing and a wire attached to the metal needle imbedded in thehub. This wire, when connected to a source, supplies electrical currentto the needle with appropriate output characteristics for generation ofaction potentials in axons. Use of this equipment requires a means forholding the needle assembly, adjusting the output current, and injectingmedication. This is awkward for a single user to accomplish and usuallyrequires the presence of an assistant.

[0006] The commercially available nerve stimulators offer two methods ofcontrolling the current output from the nerve stimulator instrument to anerve stimulator needle. The first method is by hand-operated dials onthe face of the nerve stimulator instruments. In this method, it isdifficult for a single operator to insert and position the needle in thepatient, control the current supplied the nerve stimulator needle, andinject the medication in the patient.

[0007] The second method is by a foot-operated pedal connected via acable to the nerve stimulator instrument. An output source withfoot-pedal control, such as described in U.S. Pat. No. 5,830,151 toHadzic, necessitates a multiplicity of wires connecting pieces ofequipment together. In the environment of an operating room where amultiplicity of electrical cables already exists, any equipment thatadds additional cables spread out across the floor or tables and cartsrepresents increased hazard for stumbling and consequent injury. Also,efficiency of movement is highly prized in the operating roomenvironment. When an anesthetic procedure is completed, the operatingroom personnel move rapidly to begin the surgical positioning andprepping. Often, the anesthesiologist is in the position of gathering upequipment used for a procedure, and either disposing of it or placing iton a cart for subsequent storage. Tangles of cables and wires complicatethis process and have a tendency to increase the clutter surroundinganesthesia machines and carts.

[0008] U.S. Pat. No. 4,515,168 to Chester et al. discloses to clamp anerve stimulator and locating device onto a syringe. As the entire nervestimulator device is clamped onto the syringe, the unit is a long andclumsy assembly, which is difficult to maneuver. Moreover, the devicedisclosed by Chester does not allow for one-handed operation of needleadvancement and current control. Particularly, the needle is advanced byone hand while the current must be controlled by turning the knob 27with the other hand, which is an extremely awkward operation for theuser. Additionally, the nerve stimulator of the Chester patent restrictsthe size of the syringe upon which it may be mounted, thus, requiringthe operator to change the syringe on the needle. This combination makesit very difficult to stabilize the needle within 1-2 mm of a nerve asdesired for a regional block.

[0009] U.S. Pat. No. 5,306,236 to Blumenfeld et al. discloses a handle36 to which the syringe, the needle and a conductor for carrying anelectrical signal are attached. The control mechanism for controllingthe application of current to the needle is located remotely from theneedle at a stimulator device. Like the Chester patent, the system ofBlumenfeld also does not allow for one-handed operation of both needleadvancement and current control. Accordingly, the system of theBlumenfeld patent also provides a clumsy operation for the user.

[0010] It is frequently useful, during and after the performance of aregional anesthetic procedure, to know the depth at which the nervestructure was located. The consideration of needle tip depth is valuableboth for medical record purposes as well as a check on needle positionduring the performance of a procedure. Accurate initial needle positionmay be obtained, but then undergo alteration by displacement during theinjection portion of the procedure. Displacement may be due toinadvertent pressure applied by the operator, or the tendency of theinjected fluid to force the needle back along its tissue track. Thepresence of visual guides on the needle itself, or a read out of needletip depth on the nerve stimulator device, would provide feedback to theoperator so as to prevent needle displacement. At present, there is nomechanism for providing such information with the currently availableneedles designed for use with nerve stimulators.

SUMMARY OF THE INVENTION

[0011] In accordance with the present invention, an output controlneedle comprising an insulated hypodermic needle having a means forfingertip control of the stimulator output current and a means of depthdetermination. The invention will provide a means for a single operator,the anesthetist, to accurately position a needle and perform a regionalanesthetic technique without need of an assistant or excessive cabling.This further reduces the costs associated with an assistant's time whilemaintaining operating room hazards at a minimum.

[0012] It is an object of the present invention to provide a nervestimulator needle with fingertip control of output current, which avoidsthe need for an assistant or foot pedal to adjust the output sourceduring the performance of a regional anesthetic procedure.

[0013] It is an object of the present invention to provide a nervestimulator needle with fingertip control of output current allowing thesecond hand to perform medication injection.

[0014] It is an object of the present invention to provide a nervestimulator needle with a means of electronically measuring the depth atwhich a nerve structure is encountered.

[0015] It is an object of the present invention to provide a nervestimulator needle with a means of visually measuring the depth at whicha nerve structure is encountered.

[0016] In the present invention a nerve stimulator needle apparatus maycomprise: a needle capable of carrying an electric current; a variablecontrol mechanism which is operable to variably control the amplitude ofan application of electric current to the needle; a plurality ofelectrical connectors connected to the variable control mechanism andthe needle which allows the variable control mechanism to remotelyconnect to a nerve stimulation device which is operable to provide avoltage to the variable control mechanism and to provide a current pulseto the needle having an amplitude which is controlled by the variablecontrol mechanism; and a housing which holds the variable controlmechanism, the plurality of electrical connectors and the needle.

[0017] The needle may be an insulated hypodermic needle, and theapparatus may further comprise: an injection tube operably connected tothe needle to provide a fluid to said needle. The housing may include afluid path, and the injection tube may be connected to one end of thefluid path and the needle may be connected to another other end of saidfluid path.

[0018] The variable control mechanism of the needle apparatus mayinclude a pressure sensitive switching mechanism which changes theamplitude of the current applied to the needle in relation to the amountof pressure applied to the pressure sensitive switching mechanism.

[0019] The housing of the needle apparatus may further includeconductive traces that connect the variable control mechanism and theneedle to said electrical connectors.

[0020] The needle unit of the needle apparatus may include anelectrically resistive layer covering the needle, wherein the resistanceof the resistive layer changes with the length of the resistive layer,and a conductor associated with the housing may provide a voltage to theelectrically resistive layer. The needle unit may further include aninsulating layer between the electrically resistive layer and theneedle.

[0021] The needle unit may further includes depth measurement marks thatindicate the insertion depth of the needle. The variable controlmechanism of the needle apparatus may comprise a variable opticalswitching device. The variable optical switching device may include apartially colored plate and a fiber optic cable which directs light froma light source to impinge on the partially colored plate, and directreflected light from the partially colored plate to a sensor. Thepartially colored plate preferably alters color or intensity componentof the light impinged on it from said fiber optic cable in response toan application of pressure.

[0022] The variable optical switching device may include a graduatedreflective plate and a fiber optic cable that directs light from a lightsource to impinge on the graduated reflective plate, and directreflected light from the graduated reflective plate to a sensor. Thegraduated reflective plate preferably alters color or intensitycomponent of the light impinged on it from the fiber optic cable inresponse to an application of pressure.

[0023] The invention may also include a nerve stimulator apparatuscomprising: a nerve stimulator device comprising: a voltage source; acontroller; and a current source operable to produce an electricalcurrent in response to said controller; and a needle unit remotelylocated from said nerve stimulator device and connected to said nervestimulator device only by at least one electrical conductor, said needleunit comprising: a variable control mechanism which is operable toreceive a voltage from said voltage source and to provide instructionsto said controller to variably control the amplitude of said electricalcurrent provided by said current source; a needle capable of carryingsaid electric current from said current source having the amplitudecontrolled by said variable control mechanism; and a housing which holdssaid variable control mechanism and said needle.

[0024] The variable control mechanism may include a pressure sensitiveswitching mechanism that changes the amplitude of the current applied tothe needle in relation to the amount of pressure applied to the pressuresensitive switching mechanism. The current source is preferably operableto increase or decrease current in response to the operation of thevariable control mechanism, and the variable control mechanism isoperable to increase or decrease current to the needle. The currentsource preferably changes the amplitude of the current in response tothe operation of the variable control mechanism and the variable controlmechanism is operable to control the rate of change of the current.

[0025] The needle unit may include an electrically resistive layercovering the needle, wherein the resistance of the resistive layerchanges with the length of the resistive layer, and an electrical tracefrom an electrical connector provides a voltage from the voltage source,so as to maintain a constant current, to the electrically resistivelayer. The microprocessor preferably determines the insertion depth ofthe needle according to the equation:

L _(b)=(R _(t) −R _(a))/r _(L)

[0026] where R_(a) is the resistance of the portion of the needleprotruding above a skin surface of a subject; R_(t) is the resistance ofthe total length of the needle; r_(L) is the resistance per unit lengthof the needle; and L_(b) is the insertion depth of the needle. Whereinthe value R_(a) is calculated from the ratio of the voltage of a signaldetected by a return electrode attached to the surface of the skin of asubject divided by the applied current of the resistive layer on theneedle.

[0027] The invention further comprises a method of locating nerves maycomprise the steps of: (A) inserting a needle mounted on a housingthrough a surface of skin of a subject; (B) activating a variablecontrol mechanism mounted on the housing to provide a variablycontrolled current to the needle; (C) monitoring a detected currentsignal from a return electrode attached to the surface of skin of asubject using a nerve stimulator device,

[0028] wherein steps A and B are performed by the same hand of anoperator.

[0029] The method of locating nerves may further comprise the step ofdetermining the needle insertion depth by providing a voltage to aresistive layer, completing a circuit through a return electrode on theskin surface.

[0030] The step of determining the needle insertion depth is preferablyperformed according to the equation:

L _(b)=(R _(t) −R _(a))/r _(L)

[0031] where R_(a) is the resistance of the portion of the needleprotruding above a skin surface of a subject; R_(t) is the resistance ofthe total length of the needle; r_(L) is the resistance per unit lengthof the needle; and L_(b) is the insertion depth of the needle.

[0032] The determination of the needle depth may comprise the steps of:continuously applying a constant current to a resistive layer on thesurface of the needle; periodically applying a current pulse to theneedle while applying the constant current; waiting for a period of timeto elapse after applying the current pulse to the needle and determiningthe resistance of the resistive layer of the needle exposed from thesurface of skin of the subject; and calculating the depth of the needleinserted in the surface of skin of the subject.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are incorporated in and form apart of the specification, illustrate the various embodiments of thepresent invention, and together with the description, serve to explainthe principles of the invention. In the drawings:

[0034]FIGS. 1A and 1B illustrate a first embodiment of stimulator needleassembly of the present invention;

[0035]FIG. 2 illustrates the stimulator needle assembly of the firstembodiment in operation;

[0036] FIGS. 3A-3C illustrate a second embodiment of a stimulator needleassembly of the present invention;

[0037]FIG. 4 illustrates the stimulator needle assembly of the secondembodiment in operation;

[0038]FIG. 5 illustrates a modified version of the needle shaft of thesecond embodiment;

[0039]FIG. 6 illustrates a third embodiment of a stimulator needleassembly of the present invention;

[0040]FIG. 7 is a flow diagram that illustrates the determination of theneedle depth;

[0041]FIG. 8 illustrates a fourth embodiment of a stimulator needle ofthe present invention; and

[0042]FIG. 9 illustrates the operation of the variable control mechanismof the fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] Reference will now be made in detail to the present preferredembodiment of the invention, an example of which is illustrated in theaccompanying drawings.

[0044] A preferred embodiment of the present invention comprises a nervestimulator function that allows control of the current output through afingertip control on the stimulating needle. A preferred embodiment ofthe needle assembly of the present invention is illustrated in FIG. 1A(front view) and FIG. 1B (side view). A needle assembly consists of ahousing unit 102. The housing unit may be made of any suitable materialsuch as molded plastic. The housing unit 102 preferably contains aplurality of electrical pin connectors 103, 104 and 105 thatelectrically connect the housing unit 102 to an external nervestimulator device or a plurality of external devices. Electrical traces139, 140, and 143 are preferably embedded within the housing unit 102and are electrically connected to a corresponding electrical pinconnector.

[0045] The housing unit 102 also preferably contains an embedded fluidpath 110 through which fluid from a tube 109 may flow to a needle 107.The tube 109 may be any tube suitable for carrying fluids such as aplastic injection tube. The tube 109 may be formed within the housingunit 102 or may mate with the housing unit 102 via known techniques ofmating tubes. Needle 107 is preferably inserted into a cavity (notshown) in the housing unit 102 so that it mates with the embedded fluidpath 110. Needle 107 may be detachable from housing 102 or may bepermanently affixed to housing 102. Those of skill in the art willappreciate that the tube 109 and the needle 107 should be attached tothe embedded fluid path 110 according to techniques known in the art ina manner that avoids leakage of the fluid and also avoids contaminationof the fluid. The hypodermic needle preferably consists of stainlesssteel, and is preferably coated with an insulation layer 108, although aneedle without the insulation layer may be used but may provide lessefficient current transfer to the nerve. The needle tip is preferablynot coated with the insulation layer and the needle tip may be of anytype of bevel, such as a short or a long bevel. The insulation layer 108is preferably a biocompatible insulation layer and preferably comprisedof Teflon, polyethylene, PVC, polypropylene, or any other suitablematerial.

[0046] A variable control mechanism 101 for variably controlling thecurrent applied to needle 107 may include any type of finger actionableswitch, such as a rocker switch, pressure switch, slider switch or anyother known finger actionable switch, attached to the housing unit 102.A voltage may be applied from a voltage source 180 to the variablecontrol mechanism 101 via an electrical pin connector 103 and electricaltrace 140 in the housing unit 102 and reduced or increased depending onthe operation of the variable control mechanism 101. The output voltageon the variable control mechanism 101 is directed to a control device181, via a second electrical pin connector 104, for processing anddetermining the output current to be delivered from a current source 182via the third electrical pin connector 105 to the embedded hypodermicneedle 107. The current provided by the current source 182 is preferablyin the form of a pulse train as known in the art.

[0047]FIG. 2 illustrates the operation of the stimulator needleassembly. As shown in FIG. 2, a syringe 132 is connected to housing unit102 via tube 109. A hypodermic needle 107 is also attached to thehousing unit 102, and advanced through the skin surface 136. A nervestimulator device 131 is electrically connected to electrical pinconnectors 103-105 via electrical cables 126, 142, and 122 throughelectrical pin connectors 129, 130 and 127, respectively. The source180, control device 181 and the current source 182 are contained in thenerve stimulator 131.

[0048] As the needle is advanced through the skin surface 136, the nervestimulator 131 is activated and controlled by the variable controlmechanism 101 via the current output control cable 142 attached to thenerve stimulator 131 by electrical pin connector 130. The output currentis supplied through electrical pin connector 127 via an electrical cable122 to electrical pin connector 105 on the housing unit 102. Voltage isoutput through electrical pin connector 129 via electrical cable 126 toelectrical pin connector 103 on the housing unit 102 for output currentcontrol. The electrical return electrode 135 bears a connector 134 thatattaches via an electrical cable 123 to an electrical pin connector 128on the nerve stimulator 131. The return electrode 135 is typically asilver-silver chloride electrocardiographic electrode. The operator isable to determine the proximity of the needle tip to the nerve byobservation of a visible twitch stimulated in the muscle supplied by thetarget nerve. In a nerve stimulator operation, current is supplied tothe hypodermic needle 107 at a level of 1-2 mA and the needle advanceduntil muscle twitch is achieved. By following appropriate muscletwitches, the amplitude of the current output may be gradually decreasedby the use of the variable control mechanism 101 until twitch isobserved at less than 0.5 mA as shown on display 141. When the needletip position is an appropriate distance from the nerve, e.g., 1-2 mm ata current output of less than 0.5 mA, the operator injects the solutionin the syringe. This process, using a nerve stimulator needle of theprior art, is generally described in U.S. Pat. No. 5,830,151 to Hadzicet al. A second embodiment of the present invention is illustrated inFIGS. 3A, 3B and 3C. The second embodiment is similar to the firstembodiment except that the second embodiment includes the ability todetermine the depth of the needle inserted in the skin. The samecomponents are numbered with the same reference numbers. FIG. 3Aillustrates a frontal view of the second embodiment, FIG. 3B illustratesa side view, and FIG. 3C illustrates a cross sectional view of theneedle of the second embodiment.

[0049] The needle 107 shown in FIGS. 3A, 3B and 3C is able to determinedepth of hypodermic needle penetration beneath the skin by using alinear resistance coating 106 connected to the voltage source 180 via anelectrical pin connector 103 and an electrical trace 111. The preferredmaterial for this linear resistance coating is a conductive polymercoating such as a polyaniline (Ormecon™), although any suitable linearresistive material may be used, such as a normally nonconductive polymerthat has been doped with a conductive material. For example, thenonconductive polymer silastic can be doped with carbon to becomeconductive. By controlling the amount of dopant, the resistance of thepolymer may be adjusted to suitable levels. The linear resistancecoating 106 is separated from the stainless steel hypodermic needle 107by an insulation layer 108, shown as a hatched area surrounding theneedle in the figures.

[0050] The needle of the second embodiment also may be used with thenerve stimulator 131′ shown in FIG. 4. The needle of the secondembodiment locates a nerve in the same manner as described in the firstembodiment, except that the needle of the second embodiment has theability to determine the insertion depth of the needle in the skin of asubject. For this reason, nerve stimulator 131′ in FIG. 4 contains aconstant current voltage source 190.

[0051] Existing nerve stimulators are designed to deliver constantcurrent pulses for nerve stimulation purposes. These devices do notprovide constant current (a constant non pulsed current). However, oneof skill in the art is able to readily design a constant current source190, and the circuit design may be analogous to that for producing theconstant current pulse for stimulation. The nerve stimulator 131′described herein preferably contains two separate current sources, oneof which, the constant current source 182, is adjustable by the user forstimulation pulse generation, i.e. pulse current i. The other of which,the constant current voltage source 190, is not adjustable by the userand provides a constant current (I) to the linear resistance coating106.

[0052] By way of example, as shown in FIG. 4, the return electrode 135for depth determination by resistance measurement is located remotely onthe skin surface and is the same return electrode as that for thecurrent output of the nerve stimulator 131. In operation, a voltagesignal from the constant current voltage source 190 is applied to thelinear resistance coating 106 via electrical pin connector 103 andelectrical trace 111. The circuit is completed through the linearresistance coating 106 as it penetrates the skin, thus the returnelectrode 135 detects the signal and provides the detected signal to thenerve stimulator 131′ via electrical cable 123 and electrical pinconnector 128. The nerve stimulator 131′ detects the voltage of thedetected signal using a voltmeter 145.

[0053] The resistance of the linear resistance coating 106 is constantper unit length and significantly higher than that of tissue, which ison the order of 1.0 megaohm per mm. Tissue impedance is typically in therange of 0.1-1.0 kilohms. Since the tissue impedance is less than theresistance of any portion of the needle by orders of magnitude, theresistance of the circuit is approximately that of the needle coatingalone. The total resistance (R_(t)) of the needle linear resistancecoating 106 is the product of the resistance per unit length (r_(L)) andthe length of the needle (L_(t)). As the needle is advanced through theskin, R_(t) may be represented as the sum of the length protruding abovethe skin (L_(a)) 137 multiplied by r_(L) and of the length beneath theskin (L_(b)) 138 multiplied by r_(L), as given in equation 1.

R _(t) =r _(L) ×L _(t)=(r _(L) ×L _(a))+(r _(L) ×L _(b))  Eq. 1

[0054] Therefore, since the resistance determined by the nervestimulator 131′ in this circuit is directly related to the length of thecoating that protrudes above the skin surface (R_(a)=r_(L)×L_(a)), thelength of the needle below the skin surface may be determined byequation 2.

L _(b)=(R _(t) −R _(a))/r _(L)  Eq. 2

[0055] where R_(a) is the resistance of the portion of the needleprotruding above the skin. Since R_(t) and r_(L) are known and R_(a) iscalculated directly from the ratio of the measured voltage to theapplied current (Ohm's Law: V=IR), L_(b) may be calculated. If theneedle is inserted to the point that the housing unit 102 contacts theskin surface, L_(a)=0, the measured resistance is that of the tissuealone, and thus L_(b)=L_(t) in that situation.

[0056] The return electrode 135 is used for both stimulator currentcontrol and for needle depth determination through the measurement ofresistance. FIG. 7 illustrates an exemplary method of determining thedepth of the needle which is preferably carried out by a microprocessorin the nerve stimulator 131′. As shown in step SO, the nerve stimulator131′ illustrated in FIG. 4 provides a constant DC current I, by way of avariable voltage, to the linear resistive coating 106 on the nervestimulator needle 107 via electrical pin connector 103 and electricaltrace 111. The nerve stimulator 131′ provides a periodic current pulse ito the needle 107, having its amplitude controlled by variable controlmechanism 101, via electrical trace 139 (step S2). The constant currentI is preferably continuously provided, and is not interrupted by theperiodic current pulse i provided to the needle 107. The resistanceR_(a) is determined from the DC current I after the current pulse idecays in the skin of the subject, i.e. during the portion of theinterpulse interval when no current pulse from the stimulator functionis occurring. The resistance measurement cannot occur early in theinterpulse interval due to capacitive functions of the skin which resultin a discharge voltage according to the relationshipV=V_(i)×e^((−t/RC)), where V is the observed voltage, V_(i) is theapplied voltage of the current pulse i on needle 107, t is the elapsedtime, and RC is the product of the resistance and the capacitance of theskin. Consequently, the resistance measurements for depth determinationare collected in a timed fashion, rather than continuously. For example,in the preferred embodiment, the current pulse is less than or equal to5 msec in duration and, since the frequency of the pulse signal providedto the nerve stimulator needle 107 is a maximum of 5 Hz, the interpulseduration is 45 msec.

[0057] In step S4, the microprocessor determines the decay time t of thecurrent pulse in the skin of the subject as t=5RC. Particularly, for acircuit containing a parallel resistance (R) and capacitance (C), suchas biological tissue, a tissue time constant may be calculated from theproduct of R and C. When subjected to a current pulse, the capacitiveelement of such a circuit charges over a defined time interval. Ontermination of the current pulse, this capacitive element dischargesover a characteristic time interval, which follows an exponential decaycurve, which takes about five tissue time constants (5RC) to reach 99%of the final value. This is described in more detail by Nunn (Nunn J F,Applied Respiratory Physiology, Butterworths, London, 1977, pg. 464-469)and Horowitz and Hill (Horowitz P and Hill W, The Art of Electronics,Cambridge University Press, Cambridge, Mass., 1986, pg.20-21). Sincetissue time constants are in the range of 1 msec or less, allowinggreater than 5 msec to pass following the termination of the stimulatingpulse provides adequate time for the measured voltage to approximatebaseline values of the voltage applied to the resistive layer.Alternatively, t may set to a value greater than or equal to 5 msec.

[0058] At step S6, the microprocessor waits for the time t to elapsesince the end of the current pulse i (No in step 6). Once the time t haselapsed (Yes in step 6), the measured voltage from return electrode 135via voltmeter 145 is used to calculate R_(a) (step S8). The needle depthL_(b) is then calculate in step S10 according to equation 2, anddisplayed in step S12. The calculated value of L_(b) may then bevisually displayed on display 141. Those of skill in the art willappreciate that the calculated value L_(b) may also be audibly displayedand/or may be printed by a printing device attached either directly orindirectly to nerve stimulator 131′.

[0059]FIG. 5 illustrates a cross section of the needle that may be usedwith the second embodiment. As illustrated in FIG. 5, needle 107 iscoated with insulation layer 108 that is coated with linear resistivecoating 106 as in the second embodiment. In FIG. 5, indelible marks 113,preferably comprised of biocompatible material, may be included on theresistance coating to provide visual reference of the depth to which thetip of the needle has been inserted. These depth measuring mechanismsallow accurate recording of the needle depth at which an injection wasperformed and allow for accurate maintenance of needle position duringthe injection procedure as well as for a period of time after injectionwhen subsequent stimulation attempts are undertaken by the anesthetist.It will be appreciated that the indelible marks 113 may be applied tothe insulation layer 108, in the absence of the resistive coating 106,to provide a visual depth reference.

[0060] A third embodiment is shown in FIG. 6. In this embodiment, astrip of resistance material 112, such as a tantalum wire, or ceramicstrip resistance, may be substituted for the conductive polymer. Thismaterial preferably runs the length of the needle shaft and is separatedfrom the hypodermic needle 107 by the insulation layer 108. Thismaterial provides the same linear resistance characteristics as theconductive polymer for use in depth determination, and operates in thesame manner as described in embodiment 2.

[0061] A fourth embodiment of the invention is illustrated in FIGS. 8and 9. In this embodiment, the variable control mechanism 101″ may be anoptical device controlling the current output by a mixture of lightwavelengths determined by the degree to which the variable controlmechanism 101 is depressed. An example of such a control device is theColdswitch™. In the case of optical control, the electrical pinconnector 104 is replaced by a fiber optic cable 124. The same elementsin this embodiment as in the previous embodiments are numbered the same.Just as in the first embodiment, the variable control mechanism 101″ ispreferably connected to a nerve stimulator 131 or 131′ that senses thedirection of change mediated through the variable control mechanism101″, as well as the rate of change of switching events.

[0062] The operation of this embodiment is illustrated in FIG. 9. InFIG. 9, light from an LED source 116 contained in a photonic sensor andcontrol 115 is directed via fiber optic cable 124 to an optical controlmechanism 114, contained in the variable control mechanism 101″. Opticalcontrol mechanism 114 contains a reflective plate 118 that is partiallycolored, such as by a color coating, or is a graduated reflective platethat has different reflective properties along the length of the plate.Optical control mechanism 114 also contains a pivot 119 upon whichreflective plate 118 may pivot upon the application of pressure from anoperator. The light from LED source 116 is reflected off of reflectiveplate 118 and transmitted back to the photonic sensor 117 via fiberoptic cable 124. The photonic sensor and control 115 detects thedirection of change of variable control mechanism 101″, mediated throughthe optical control mechanism 114, as well as the rate of change ofswitching events, by the reflected color or intensity mixing. Color orintensity mixing is accomplished by a reflective plate 118. When thereflective plate 118 is rocked by finger pressure on its pivot 119, thewhite incident light emitted from the fiber optic cable 124 is reflectedback with altered color or intensity components. The intensity or coloris detected by the photonic sensor 117, electronically processed by themicroprocessor 120 and converted to a corresponding current output viathe constant current pulse generator 121. The current output is directedto the stimulating needle electrical pin connector 105 by an electricalcable 122 with the return supplied by a second electrical cable 123 tothe return electrode 135 located remotely on the skin surface. Just asin the first embodiment, commands to the nerve stimulator can becontrolled through switching events, including, but not limited tosequential taps, sudden release or sudden depression.

[0063] The foregoing description of a preferred embodiment of theinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. For example, although the method of FIG. 7 is described asbeing performed by a microprocessor, the method may be performed by ahard wired system or any other suitable processing system. Additionally,the terms electrical traces and electrical cables is are considered toencompass any type of known electrical conductors, such as, but notlimited to metallic wires or non metallic electrical conductors, whichmay be embedded or non-embedded, and which may be coated with aninsulator or non-coated.

What is claimed is:
 1. A nerve stimulator needle apparatus comprising: aneedle capable of carrying an electric current; a variable controlmechanism which is operable to variably control the amplitude of anapplication of electric current to said needle; a plurality ofelectrical connectors connected to said variable control mechanism andsaid needle which allows said variable control mechanism to remotelyconnect to a nerve stimulation device which is operable to provide avoltage to said variable control mechanism and to provide a currentpulse to said needle having an amplitude which is controlled by saidvariable control mechanism; and a housing which holds said variablecontrol mechanism, said plurality of electrical connectors and saidneedle.
 2. The nerve stimulator needle apparatus of claim 1, whereinsaid needle is a hypodermic needle, said apparatus further comprising:an injection tube operably connected to said needle to provide a fluidto said needle.
 3. The nerve stimulator needle apparatus of claim 2,wherein said housing includes a fluid path, and said injection tube isconnected to one end of said fluid path and said needle is connected toanother other end of said fluid path.
 4. The nerve stimulator needleapparatus of claim 1, wherein said variable control mechanism includes apressure sensitive switching mechanism which changes the amplitude ofsaid current applied to said needle in relation to the amount ofpressure applied to said pressure sensitive switching mechanism.
 5. Thenerve stimulator needle apparatus of claim 1, wherein said variablecontrol mechanism is operable to increase or decrease current to theneedle.
 6. The nerve stimulator needle apparatus of claim 1, whereinsaid variable control mechanism is operable to control a rate of changeof the current.
 7. The nerve stimulator needle apparatus of claim 1,wherein said housing further includes conductive traces which connectsaid variable control mechanism and said needle to said electricalconnectors.
 8. The nerve stimulator needle apparatus of claim 1, whereinan electrically resistive layer covers said needle, the resistance ofsaid resistive layer changes with the length of said resistive layer,and an electrical trace from an electrical pin connector provides avoltage to said electrically resistive layer from said nerve stimulatordevice.
 9. The nerve stimulator needle apparatus of claim 8, whereinsaid needle unit further includes an insulating layer between saidelectrically resistive layer and said needle.
 10. The nerve stimulatorneedle apparatus of claim 1, wherein said needle unit further includesdepth measurement marks which indicate the insertion depth of saidneedle.
 11. The nerve stimulator needle apparatus of claim 1, whereinsaid variable control mechanism comprises a variable optical switchingdevice.
 12. The nerve stimulator needle apparatus of claim 11, whereinsaid variable optical switching device includes a partially coloredplate and a fiber optic cable which directs light from a light source toimpinge on said partially colored plate, and directs reflected lightfrom said partially colored plate to a sensor.
 13. The nerve stimulatorneedle apparatus of claim 12, wherein said partially colored platealters color or intensity components of the light impinged on it fromsaid fiber optic cable in response to an application of pressure. 14.The nerve stimulator needle apparatus of claim 11, wherein said variableoptical switching device includes a graduated reflective plate and afiber optic cable which directs light from a light source to impinge onsaid graduated reflective plate, and directs reflected light from saidgraduated reflective plate to a sensor.
 15. The nerve stimulator needleapparatus of claim 14, wherein said graduated reflective plate alterscolor or intensity components of the light impinged on it from saidfiber optic cable in response to an application of pressure.
 16. Thenerve stimulator needle apparatus of claim 1, wherein said needle is aninsulated needle.
 17. A nerve stimulator apparatus comprising: a nervestimulator device comprising: a voltage source; a controller; and acurrent source operable to produce an electrical current in response tosaid controller; and a needle unit remotely located from said nervestimulator device and connected to said nerve stimulator device only byat least one electrical conductor, said needle unit comprising: avariable control mechanism which is operable to receive a voltage fromsaid voltage source and to provide instructions to said controller tovariably control the amplitude of said electrical current provided bysaid current source; a needle capable of carrying said electric currentfrom said current source having the amplitude controlled by saidvariable control mechanism; and a housing which holds said variablecontrol mechanism and said needle.
 18. The nerve stimulator apparatus ofclaim 17, wherein said needle is a hypodermic needle, said apparatusfurther comprising: an injection tube operably connected to said needleto provide a fluid to said needle.
 19. The nerve stimulator apparatus ofclaim 18, wherein said housing includes a fluid path, and said injectiontube is connected to one end of said fluid path and said needle isconnected to another other end of said fluid path.
 20. The nervestimulator apparatus of claim 17, wherein said variable controlmechanism includes a pressure sensitive switching mechanism whichchanges the amplitude of said current applied to said needle in relationto the amount of pressure applied to said pressure sensitive switchingmechanism.
 21. The nerve stimulator apparatus of claim 17, wherein saidcurrent source is operable to increase or decrease current in responseto the operation of said variable control mechanism, and said variablecontrol mechanism is operable to increase or decrease current to theneedle.
 22. The nerve stimulator apparatus of claim 17, wherein thecurrent source changes the amplitude of said current in response to theoperation of said variable control mechanism and said variable controlmechanism is operable to control the rate of change of the current. 23.The nerve stimulator apparatus of claim 17, further comprisingelectrical connectors mounted to said housing, wherein the electriccurrent is provided through said electrical connectors.
 24. The nervestimulator apparatus of claim 23, wherein said housing further includesconductive traces which connect said variable control mechanism and saidneedle to said electrical connectors.
 25. The nerve stimulator apparatusof claim 17, wherein said needle unit includes an electrically resistivelayer covering said needle, wherein the resistance of said resistivelayer changes with the length of said resistive layer, and an electricaltrace from an electrical connector provides a voltage from said voltagesource to said electrically resistive layer.
 26. The nerve stimulatorapparatus of claim 25, wherein said needle unit further includes aninsulating layer between said electrically resistive layer and saidneedle.
 27. The nerve stimulator apparatus of claim 17, wherein saidneedle unit further includes depth measurement marks that indicate theinsertion depth of said needle.
 28. The nerve stimulator apparatus ofclaim 25, wherein said controller includes a microprocessor, and saidmicroprocessor determines the insertion depth of the needle according tothe equation: L _(b)=(R _(t) −R _(a))/r _(L) where R_(a) is theresistance of the portion of the needle protruding above a skin surfaceof a subject; R_(t) is the resistance of the total length of the needle;r_(L) is the resistance per unit length of the needle; and L_(b) is theinsertion depth of the needle.
 29. The nerve stimulator apparatus ofclaim 25, wherein the value R_(a) is calculated from the ratio of theapplied voltage to the resistive layer on the needle divided by thecurrent detected by a return electrode attached to the surface of theskin of a subject.
 30. The nerve stimulator apparatus of claim 17,wherein said variable control mechanism comprises a variable opticalswitching device.
 31. The nerve stimulator apparatus of claim 30,wherein said variable optical switching device receives light from alight source and includes a partially colored plate and a fiber opticcable which directs the light from said light source to impinge on saidpartially colored plate, and directs reflected light from said partiallycolored plate to a sensor.
 32. The nerve stimulator apparatus of claim31, wherein said partially colored plate alters color or intensitycomponents of the light impinged on it from said fiber optic cable inresponse to an application of pressure, and said sensor converts saidaltered color or intensity components to a corresponding current signal.33. The nerve stimulator apparatus of claim 30, wherein said variableoptical switching device includes a graduated reflective plate and afiber optic cable which directs light from a light source to impinge onsaid graduated reflective plate, and directs reflected light from saidgraduated reflective plate to a sensor.
 34. The nerve stimulator needleapparatus of claim 33, wherein said graduated reflective plate alterscolor or intensity components of the light impinged on it from saidfiber optic cable in response to an application of pressure.
 35. Thenerve stimulator apparatus of claim 17, wherein said needle is aninsulated needle.
 36. A method of locating nerves comprising the stepsof: (A) inserting a needle mounted on a housing through a surface ofskin of a subject; (B) activating a variable control mechanism mountedon said housing to provide a variably controlled current to said needle;(C) monitoring a detected current signal from a return electrodeattached to said surface of skin of a subject using a nerve stimulatordevice, wherein steps A and B are substantially performed simultaneouslyby the same hand of an operator.
 37. The method of locating nervesaccording to claim 36, further comprising the step of determining theneedle insertion depth by providing a voltage to a resistive layer,completing a circuit through a return electrode on the skin surface. 38.The method of locating nerves according to claim 37, wherein the step ofdetermining the needle insertion depth is performed according to theequation: L _(b)=(R _(t) −R _(a))/r _(L) where R_(a) is the resistanceof the portion of the needle protruding above a skin surface of asubject; R_(t) is the resistance of the total length of the needle;r_(L) is the resistance per unit length of the needle; and L_(b) is theinsertion depth of the needle.
 39. The method of locating nervesaccording to claim 38, wherein the value R_(a) is calculated from theratio of the applied voltage to the resistive layer on the needledivided by the current detected by the return electrode attached to thesurface of the skin of a subject.
 40. The method of locating nervesaccording to claim 36, wherein the determination of the needle depthcomprises the steps of: continuously applying a constant current to aresistive layer on the surface of said needle; periodically applying acurrent pulse to said needle while applying said constant current;waiting for a period of time to elapse after applying said current pulseto said needle and determining the resistance of the resistive layer ofsaid needle exposed from the surface of skin of the subject; andcalculating the depth of said needle inserted in the surface of skin ofthe subject.